The present invention relates to the frame of a game racket such as a tennis racket, badminton racket, or squash racket, and more particularly to such a frame of a game racket formed from fiber reinforced thermoplastic materials.
In the past the frame of a game racket was formed exclusively from wood. However, in order to obtain higher performance of the racket on the one hand, and on the other hand in order to avoid problems such as the difficulty in obtaining high quality wood material and the technical limits with regard to manufacturing wooden rackets of highly uniform quality, in recent years gradually increasing numbers of rackets have been made from other materials.
The materials which are now taking the place of wood to construct the frame of a game racket are metals and fiber reinforced plastics (FRP). Particularly, fiber reinforced thermoplastic (FRTP) is now commonly considered because with injection moulding it allows high productivity and high uniformity of quality. However, glass fiber reinforced thermoplastic which employs glass fibers as the reinforcing fiber material and is available at a relatively low cost, is much more flexible than wood, and therefore if a racket frame is made from this material so as to have the same rigidity as a wooden frame, it becomes too heavy to be suitable for practical use. On the other hand, carbon fiber reinforced thermoplastic employing carbon fibers as the reinforcing material has a relatively high rigidity due to the relatively high elastic coefficient of carbon fibers and is used in practice as the material for constructing the frame of a game racket. However carbon fiber reinforced thermoplastic is relatively expensive, and yet a racket frame made from this material is inferior to a wooden frame with regard to the relation between weight and rigidity.
The abovementioned problem with regard to lower rigidity relative to weight is due to the fact that it is difficult or almost impossible to construct the looped head portion of the racket frame as a hollow structure by moulding or particularly injection moulding of fiber reinforced thermoplastic. In view of this, various composite frame structures assembled from a plurality of members each having a shape available from moulding of fiber reinforced thermoplastic and having as a whole a hollow structure so as to increase its rigidity relative to its weight, have been proposed, as disclosed, for example, in U.S. Pat. Nos. 3,647,211; 3,690,658; 3,840,230; 3,889,951; 3,949,988 and 3,993,908. However, the composite racket frames proposed in these U.S. patents have relatively complicated structures requiring a relatively large number of parts and are believed to be relatively expensive.